Atmospheric profile and precipitation properties derived from radar and radiosondes during RICO Louise Nuijens With thanks to: Bjorn Stevens (UCLA) Margreet van Zanten & Pier Siebesma (KNMI) September

In this talk Analysis of S-Pol radar data How frequent and with what intensity do trade wind cumuli produce rain?  from a large scale perspective Analysis of sounding data Can we relate this rainfall to the vertical (dynamic) structure of the atmosphere? Can we provide statistics for modeling studies?

Radar analysis: S-Pol  S-Band horizontal (surveillance) scans  Time resolution: ~ 70 scans a day  a scan each 20 minutes  Filtering of ground / sea clutter, birds and Bragg scatter (10 dBZ threshold)  Z-R relation: Uijlenhoet, 2004  1 mm/h  24 dBZ > 10 dBZ 20 km 100 km RawFiltered

Data selection based on range attenuation 20 km - 60 km z ~ 500 m The radar observes rain below cloud base (~ 600 m)

Area rainfall during RICO: Time series and probability of rainfall

Time series of area rainfall many small rain peaks “trade cu period” Mean echo coverage: 1.7 % Mean rainrate mm/h ~ 18 W/m2

Rainfall probability 50 % How much do the small peaks contribute to the total rainfall? mm/h

Echo structure and rain rates Coverage = 2.55 % R echo = 2.12 mm/h Echo cells (3 km) Dec 22, 05 UTC Coverage = 1.41 % R echo = 1.52 mm/h Rainband (> 20 km) Dec 29, 01 UTC

Can we relate the relative amount of rainfall to the atmospheric profiles ?

Three type of soundings: 1)Spanish Point on Barbuda 2)R/V Seward Johnson 3)C-130 dropsondes Sounding analysis

NE radar domain  q SST

Sounding analysis Do soundings relate to small rain events? Relate each sounding to a 6 hour average area rainrate Group by following rainrates in mm/h : 1.0 – ~ 1.4 Wm – 0.02~ 15 Wm -2 3.> 0.02 NE Dec 13, 14, 15 Jan 9, 13, 14, 18

Vertical profiles Specific humidity 1.5 g/kg >0.02

Vertical profiles Temperature

Vertical profiles Relative humidity %

Rainfall versus cloud depth Estimating cloud depth for each sounding Using a very simple parcel ascent scheme for a diluting air parcel: Air parcel released from max  v in the lower subcloud layer Giving small release excesses  l and  q t With entrainment rate inversely proportional to height of parcel Cloud base = LCL, cloud top  LZB (  v, parcel =  v )

Rainfall versus cloud depth

Some last thoughts… How do the LES simulations and profiles compare to these findings? Difficult to obtain a close correlation between parameters derived from soundings and the area rainfall –water vapor deficiency? –theta gradient in cloud layer? –…?

Random / scattered

Clusters?

Deeper convection

Comparison to other estimates Area rain rate [mm/h] S-Pol rain rate coincides with relative rain rate from surface station. Suggests that rain reaches the surface.